The present invention relates in general to power converters including inverters for an electric drive system of an electrified vehicle, and, more specifically, to selectively inserting a dead-time for controlling switching devices to avoid shoot-through without introducing any significant distortion in the output of the converter.
Electric vehicles, such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and battery electric vehicles (BEVs), use inverter-driven electric machines to provide traction torque and regenerative braking torque. A typical electric drive system includes a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main DC linking capacitor. An inverter is connected between the main buses for the DC link and a traction motor in order to convert the DC power to an AC power that is coupled to the windings of the motor to propel the vehicle. A second inverter may also be connected between the main bus and a generator (if present) to provide another power flow path from a prime mover, typically an internal combustion engine, to the DC link.
The inverters include transistor switching devices (such as insulated gate bipolar transistors, or IGBTs) connected in a bridge configuration including a plurality of phase legs. A typical configuration includes a three-phase motor driven by an inverter with three phase legs. An electronic controller turns the switches on and off in order to invert a DC voltage from the bus to an AC voltage applied to the motor, or to rectify an AC voltage from the generator to a DC voltage on the bus. In each case, the inverters are controlled in response to various sensed conditions including the rotational position of the electric machine and the current flow in each of the phases.
The inverter for the motor may preferably pulse-width modulate the DC link voltage in order to deliver an approximation of a sinusoidal current output to drive the motor at a desired speed and torque. Pulse Width Modulation (PWM) control signals applied to the gates of the IGBTs turn them on and off as necessary so that the resulting current matches a desired current.
Because each phase leg of the inverter has a pair of upper and lower switching devices connected across the DC link, it is important that both devices not be conducting (i.e., turned-on) simultaneously. Otherwise, the resulting “shoot-through” of the phase leg could result in damage to the switching devices. A short time interval during which both the upper and lower switching devices of a phase leg are turned off, known as a dead-time, is typically used in connection with PWM control of inverters in order to prevent shoot-through. In typical PWM control, a comparison of a PWM duty cycle with a PWM carrier signal identifies a transition time at which the output junction of a phase leg switches from being connected from one side of the DC link (i.e., positive or negative) to the other. Without dead-time insertion, one gate signal for the phase leg would change from On to Off at the same time that the other gate signal would change from Off to On. The most common form of dead-time insertion adds dead-time on both sides of the transition time, e.g., causes the “On” gate signal to go “Off” prior to the transition time and briefly delays the change of the “Off” gate signal to the “on” state. However, the insertion of dead-time has resulted in distortion of the output waveform delivered to the load and the introduction of control delays.
Efforts to avoid or reduce distortion by modifying the dead-time insertion have involved other penalties. For example, compensation circuits have been added to the switching devices in the phase legs which has resulted in increases in cost and manufacturing complexity. Other efforts have been directed to modifications to the inserted dead-times according to the direction of current flow within a particular phase leg, which has also resulted in additional cost and complexity.